code: def check_step_names(graph):
      msg = (
          "Step *{0.name}* has an invalid name. Only lowercase ascii "
          "characters, underscores, and digits are allowed."
      )
      for node in graph:
          if re.search("[^a-z0-9_]", node.name) or node.name[0] == "_":
              raise LintWarn(msg.format(node), node.func_lineno)
  
imports: import re
  from .exception import MetaflowException
  from .util import all_equal
label_desc: Use image node_id 4 for calling a global function with example usage: check_step_names(graph) without return types